Method and apparatus for microwave phosphor synthesis

ABSTRACT

A method of microwave synthesis of phosphors includes a microwave furnace having a microwave chamber; providing starting material in the microwave chamber; and subjecting the starting material to microwaves, whereby the starting material is synthesized into phosphors. An insulation package for use in microwave synthesis is disclosed that includes an insulator having an opening therein, wherein the opening is substantially symmetrically disposed in relation to a central axis of the insulator, and wherein the opening is adapted to receive starting material. A susceptor configuration may be positioned within the cavity, wherein the insulator, the cavity, and the susceptor configuration are substantially symmetrically disposed in relation to an axis of rotation of the insulation package. A microwave furnace for continuous microwave synthesis of phosphors is also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to microwave processing and, morespecifically, to a method of and apparatus for phosphor synthesis.

2. Description of Related Art

Microwave processing has been conducted on various substances (i.e.,starting materials) including ceramics, composites, cermets, hardmetals, electronic ceramics, metallic materials, etc. The generalfeatures of microwave processing of materials include volumetric andselective heating, enhanced reaction kinetics, the potential to improveproduct quality, process simplification, and the potential of costreduction.

Microwave processing of ceramic materials is a dielectric heatingprocess. The mechanism of microwave heating is inherently different fromthat of conventional heating. In microwave heating, heat is generatedwithin the materials exposed to the microwave field throughmicrowave-material interactions, whereas during conventional heating,heat is transferred from the heating element to the surface of the loadby radiation and convection, then to the center of the load by thermalconduction. The absorption of microwave energy by the load in amicrowave cavity depends on the dielectric loss factor of the materialsin the microwave field. For the highly lossy materials, microwaveprocessing can bring about substantial savings in time and energy withimproved quality of the product. For example, regular, porous, andtransparent hydroxapatite (HAp) ceramics have been fabricated bymicrowave processing within a few minutes; Ba(Zn_(1/3)Ta_(2/3))O₃ hasbeen sintered to full density by microwave processing within 30 minutesat 1300–1400° C. compared to conventional sintering of the same materialthat requires 1600° C. and as long as 24 hours.

With reference to synthesis of fluorescent lamp phosphors, in order foractivators to be incorporated into the crystal lattice structure of ahost material, a high-temperature thermal treatment is necessary.Conventional processing of fluorescent lamp phosphors includes blendingof the starting materials, loading the mixtures into crucibles, andfiring at a high temperature for several hours. Additional finishingsteps may include milling, washing to remove residual materials,filtering, drying, and blending. Although flux may be used to lower thefiring temperature and accelerate the synthesis, the time length forsynthesizing fluorescent lamp phosphors via conventional processing maystill last anywhere from 8–12 hours. Further, contamination due to thevolatiles from the conventional process can be of a concern.Additionally, the resultant phosphor obtained from a conventionalprocess is a hard caked substance, thereby requiring the need to grindup the phosphor prior to utilization. As has been described, theconventional process of phosphor synthesis is not only complex, but alsorequires significant time and energy.

Therefore, it would be an advantage to lower the complexity and amountof time and energy utilized in obtaining fluorescent phosphors.

SUMMARY OF THE INVENTION

The present invention addresses a microwave processing method andapparatus for optimizing the synthesis of phosphors and in particular,fluorescent lamp phosphors. Specifically, the present inventionsubstantially enhances the kinetics of phosphor synthesis throughmicrowave processing. Namely, the soaking time at the final temperaturein the microwave process is reduced by up to 90% of the time needed in aconventional process. In addition to the time and energy savings,microwave processing makes it possible to synthesize high qualityphosphors without using any flux, thereby reducing contamination andlowering operating costs. Microwave processing causes the resultantphosphor to assume a fine powder property, which therefore, reduces theamount of steps necessary to reduce the phosphor into a usable form. Theoptimal fluorescent lamp phosphors that may be produced by the presentinvention may be white, clean, loose, fine powders.

An insulation package for microwave synthesis of phosphors includes aninsulator having an opening therein, wherein the opening issubstantially symmetrically disposed in relation to a central axis ofthe insulator, and wherein the opening is adapted to receive startingmaterial. A susceptor configuration may be positioned within the cavity,wherein the insulator, the cavity, and the susceptor configuration aresubstantially symmetrically disposed in relation to an axis of rotationof the insulation package.

A microwave furnace for continuous microwave synthesis of phosphorsincludes a tube for receiving starting material, wherein the tubeincludes a first end and a second end; an insulator having athroughbore, wherein the tube extends therethrough; a microwave chamberfor receiving the tube and the insulator, wherein the tube, theinsulator, and the microwave chamber are in substantially parallelrelation to each other; and at least one microwave head for directingmicrowaves into the microwave chamber.

Any microwave utilized in the present invention may be configured toprovide a user-defined atmospheric environment, wherein the atmosphericenvironment includes one or more of a reduction in atmospheric pressurein relation to the atmospheric pressure outside of the microwavechamber; an increase in atmospheric pressure in relation to theatmospheric pressure outside of the microwave chamber; and anintroduction of one or more gases into the microwave chamber.

A method for obtaining phosphors through microwave synthesis is alsoincluded. The present invention may be utilized for synthesis of variousphosphors including, but not limited to halophosphate, barium-magnesiumaluminate, lanthanum phosphate, and europium doped yttrium oxide. Themethod of microwave synthesis may encompass the use of the insulationpackage and the microwave furnace. Optimal phosphors may be synthesizedif the starting material is deposited into a low microwave absorbinginsulator of particular geometric design and then subjected to an evendistribution of microwave radiation. Specifically, the insulatorincludes an opening that is substantially symmetrically disposed inrelation to a central axis of the insulator. This geometric design, inaddition to an optional susceptor configuration or other heat managementobject or device, is conducive to obtaining optimal synthesizedphosphors.

These and other advantages of the present invention will be understoodfrom the description of the desirable embodiments, taken with theaccompanying drawings, wherein like reference numbers represent likeelements throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side cross-sectional view of an insulation package on amicrowave turntable in a batch processing configuration in accordancewith the present invention;

FIG. 2 is a top view of the insulation package of FIG. 1 according to afirst embodiment along the lines II—II;

FIG. 3 is a top view of the insulation package of FIG. 1 according to asecond embodiment along the lines III—III;

FIG. 4 is a side cross-sectional view of a microwave furnace in acontinuous processing configuration in accordance with the presentinvention;

FIG. 5 is a graph showing X-ray diffraction (XRD) patterns of microwavedBAM and a conventional BAM (SBAM-1-HF1) phosphor;

FIG. 6 is a graph showing thermal stability of microwave-synthesized LAPphosphors;

FIG. 7 is a graph showing x-ray diffraction patterns ofmicrowave-synthesized unfluxed YOE phosphors in comparison with controlYOE-ML materials, indicating the identical phase composition; and

FIG. 8 is a graph showing the emission of microwave-synthesized YOEphosphors.

DETAILED DESCRIPTION OF THE INVENTION

An insulation package 10 for use in batch microwave synthesis ofphosphors is illustrated in FIGS. 1–3. As depicted in FIG. 1, theinsulation package 10 desirably includes an insulator 12 and one or moresusceptors 14. The insulator 12 includes an opening, such as cavity 16,adapted to support a crucible 18 having a starting material 20 therein.In the context of microwave synthesis, the insulation package 10 may bepositioned onto a turntable 22 of a microwave furnace (not shown). Incontrast to the batch microwave synthesis afforded by the insulationpackage 10, FIG. 4 illustrates a microwave furnace 24 adapted forcontinuous microwave synthesis. The configuration conducive to optimalsynthesis of phosphors, as embodied in the insulation package 10, isalso embodied within the construction of the microwave furnace 24.

Returning to FIG. 1, the insulator 12 of the insulation package 10 isdesirably constructed of insulating material, such as aluminosilicatefibers, alumina fibers, or boron nitride fibers, that is porous instructure, lightweight, chemically stable, thermally stable, hasexcellent thermal insulation, and has low microwave absorptionproperties. It is to be understood that the type of insulating materialutilized is dependent on the required processing conditions, especiallytemperature range and atmosphere. Desirably, the refractory materialutilized for the insulator 12 is essentially transparent to microwavesin the working temperature range. One such insulating material is knownas FiberFrax®, manufactured by Unifrax Corporation of Niagara Falls,N.Y.

Desirably, the insulation package 10 is of a symmetrical design relativeto an axis of rotation, which in this embodiment is a vertical axis 26.Thus, the insulator 12, the susceptors 14, and the cavity 16 aresymmetrically disposed in relation to the rotational axis or verticalaxis 26. Thus, as shown in FIGS. 1–3, the geometrical configuration ofthe insulator 12 is cylindrical and the geometrical arrangement of thesusceptors 14 is cylindrical. The axial symmetry of the insulatorpackage 10 allows the starting material 20 to be uniformly radiated bymicrowaves through the rotation about the vertical axis 26 via theturntable 22. The symmetry ensures uniform temperature distribution,which is conducive to the optimal synthesis of phosphors. The insulator12 may include an opening 28 that permits a temperature measuringdevice, such as a pyrometer (not shown), to measure the temperature ofthe workload within the cavity 16.

It is to be understood that the insulation package 10 may embody variousconfigurations conducive to optimal microwave synthesis of phosphors.Each configuration may include a different layout or form of the one ormore susceptors 14. As is known in the art, a susceptor has multiplefunctions, including partitioning microwaves, adjusting energydistribution, and compensating heat loss from a surface of an object.The use of the susceptors 14 in the context of the present invention isto improve microwave heating efficiency and achieve uniform microwaveheating of the starting material 20. It is to be understood that it isimportant to achieve uniform temperature distribution for optimalmicrowave heating. Thus, the geometric configuration of the insulator 12and the susceptor 14 are important for obtaining uniform heating.

For batch processing, the use of one or more susceptors 14 will allowfor more efficient microwave heating. However, it is to be understoodthat the combination of the type, amount, and configuration of one ormore susceptors 14 is important. For example, silicon carbide (SiC) isconsidered as a good susceptor 14, yet cannot be used at hightemperatures in reducing atmosphere, as an SiC susceptor will be reducedto silicon. Due to most ceramic materials having very little microwaveabsorption at low temperatures, using an appropriate susceptor will helppreheat the starting material to such a temperature range that thestarting material 20 will begin absorbing microwaves efficiently. It isto be understood, that the partitioning of the microwave energy betweenthe starting material 20 and the susceptor 14 is important. The greaterthe amount of the susceptor 14 in relation to the starting material 20,the less microwave energy is available to the starting material and,therefore, microwave radiation to the starting material 20 may bereduced. Desirably, in order to achieve an optimal microwave effect, theamount of susceptor 14 should be limited to a minimum amount necessary.

Thus, there are various configurations of susceptors that may beutilized in the present invention. For example, a first embodimentinsulation package 10 a is depicted in FIG. 2. The first embodimentinsulation package 10 a includes a plurality of susceptor rods 14 aarranged in a circular configuration within the cavity 16. Desirably,each of the susceptor rods 14 a is longer than the crucible 18 in orderto compensate for heat loss from the top and bottom of the cavity 16.Desirably, each susceptor rod of the first embodiment insulation package10 a is constructed of composite granular susceptors encased in aluminatubing, or other suitable tubing including, but not limited to mullite,fused quartz, MgO, and ZrO₂. The granular susceptors may be SiC grains,SiC—Al₂O₃ mixture, zirconia, MoSi₂, ferrite, or other microwaveabsorbing materials. In an alternate embodiment (not shown) of the firstembodiment insulation package 10 a, each of the susceptor rods 14 a maybe constructed as unitary sintered susceptor rods which do notnecessarily include any encasing tubing.

With reference to FIG. 3, and with continuing reference to FIG. 2, asecond embodiment insulation package 10 b is shown. Similar to the firstembodiment insulation package 10 b, composite granular susceptors 14 bmay be encased by an inner alumina tube 30 a and an outer alumina tube30 b. Either the inner alumina tube 30 a, the outer alumina tube 30 b,or both may be constructed of other suitable material including, but notlimited to mullite, fused quartz, MgO, and ZrO₂. A comparison of FIG. 2to FIG. 3, shows that the overall difference between the firstembodiment insulation package 10 a and the second embodiment insulationpackage 10 b, is the susceptor configuration. The second embodimentinsulation package 10 b substitutes the circular equidistant multiplesusceptor rod configuration with a cylindrical susceptor configuration.Use of one susceptor configuration over another susceptor configurationis dictated by the synthesis application. For example, due to theconfiguration of the susceptors 14 a, the first embodiment insulationpackage 10 a allows more microwaves to directly radiate to the startingmaterial 20 and, therefore, the preferred embodiment of the presentinvention is the first embodiment insulation package 10 a. The secondembodiment insulation package 10 b having the composite granularsusceptors 14 b encased within the inner and outer alumina tubes 30 a,30 b may be utilized when sensitive starting material 20 is subjected tomicrowaves or when susceptor rods 14 a are not readily available.

The crucible 18 for use with the insulation packages 10 a, 10 b may beconstructed of alumina or any other suitable ceramic material. Thecrucible 18 is dimensioned so that the starting material 20 is preventedfrom forming a “hot spot” within the center of the starting material 20.For example, if the crucible 18 is too thick, thermal runaway may occurif the center of the starting material 20 becomes too hot. Additionally,the crucible 18 is sized to fit within the cavity 16.

The microwave furnace for use with the insulation packages 10 a, 10 bmay be a 6 kW batch microwave multimode system operating at 2.45 GHz.The microwave furnace may also be equipped with a vacuum pump and aproper atmospheric controlling system (both not shown) for maintaining adesired atmosphere within the insulation package 10 as well as the wholemicrowave chamber. Thus, the microwave synthesis may be conducted undervarious controlled atmospheric conditions.

With reference to FIG. 4, and with continuing reference to FIGS. 1–3,the microwave furnace 24 for continuous microwave synthesis is depicted.The microwave furnace 24 includes a starting material inlet 32 desirablyin fluid communication with a powder outlet 34 via a rotatable tube,such as an alumina tube 36, situated therebetween. It is to beunderstood that the rotatable tube may be constructed of other suitablematerial. Insulation 38, similar in construction to the insulator 12 ofthe insulation packaging 10, envelopes an outer diameter of the aluminatube 36. Desirably, the alumina tube 36 extends through an openingextending through a central axis of the insulation 38, such as athroughbore 39. It is to be understood that the alumina tube 36 and theinsulation 38 are of a symmetrical design, similar to the geometricsymmetry embodied by the insulation package 10. The alumina tube 36 andthe insulation 38 are situated within a microwave chamber 40. One ormore microwave heads 42 are configured to produce microwaves and todirect the microwaves into the microwave chamber 40. The microwavefurnace 24 may include a water cooling apparatus 44 for preventing themetallic wall of the microwave chamber and the ceramic tube fromoverheating. A gas inlet 46 for transmitting gas therethrough isconnected to the starting material inlet 32. Specifically, the gas inlet46 is configured to direct the gas against or through the incomingstarting material 20 to cause the starting material 20 to enter thealumina tube 36. A rotary motor 48 imparts rotation upon the aluminatube 36.

In operation, the starting material 20 is continuously fed from one endof the alumina tube 36 and is continuously discharged from the other endof the alumina tube 36. The starting material inlet 32 or one end of thealumina tube 36 of the microwave furnace 24 may be raised to tilt themicrowave furnace 24 to facilitate the movement of the starting material20 through the alumina tube 36 to the powder outlet 34. Due to therotation of the tilted alumina tube 36 by the rotary motor 48, thestarting material continually moves inside the alumina tube 36 as it issynthesized into a phosphor. Thus, the alumina tube 36 is the functionalequivalent of the crucible 18 in the batch processing configurationshown in FIG. 1. Note that the microwave furnace 24 having a continuousprocessing configuration adapted for continuous microwave synthesis doesnot have any susceptors 14. The reasoning for the absence of thesusceptors 14 is that the starting material 20 is continuously fedthrough the microwave furnace 24 during processing and, therefore, thestarting material 20 will be gradually preheated by the heat dissipatedfrom the “hot zone” before entering the “hot zone.” Specifically, uponinitial start-up of the microwave furnace 24, a sufficient amount oftime is required until the microwave furnace 24 is fully heated.Thereafter, the operation is in a stable condition and the zonedistribution becomes fixed. However, this does not mean that susceptorsmay never be utilized in the continuous processing configuration, assusceptors may be used for energy distribution adjustment purposes.

An exemplary embodiment microwave processing procedure for phosphorsynthesis will now be discussed. Various lamp phosphors were synthesizedby the microwave processing technique of the present invention in a 6 kWbatch multimode microwave furnace operating at 2.45 GHz. The phosphorsincluded Ca₁₀(PO₄)₆(Cl,F):Sb:Mn (Halophosphate); Y₂O₃:Eu (YOE oreuropium doped yttrium oxide), BaMgAl₁₀O₁₇ by itself or phase mixturesof BaMgAl₁₀O₁₇ plus MgAl₂O₄ and Al₂O₃, or some product where thechemical formula is BaMg_(1+x)Al_(10+y)O_(17+z) where the x, y, and znumbers are 0<x<2.0, 0<y<5.0, and 0<z<10.5 (BAM or barium-magnesiumaluminate); and (La,Ce,Tb)PO₄:Ce:Tb (LAP or lanthanum phosphate).Phosphors were prepared both with and without flux. In order tofacilitate heat pickup at low temperatures, the microwave susceptor 14was used. The temperature was measured with an optical pyrometer throughthe opening 28.

The general procedure started with placing 30–70 grams of startingmaterial into an alumina crucible covered with a lid. The startingmaterial was in the form of fine powder. If a special atmosphere wasrequired, the microwave chamber was evacuated to 10 torr before fillingin with the required gas. The starting material was then heated bymicrowave irradiation. The heating rate and the temperature werecontrolled through proper adjustment of the power level to a magnetron.Once the desired temperature was reached, the loaded material was soakedfor a specified period of time and then cooled down by switching off themicrowave power.

The microwave-synthesized products were characterized for particle size,brightness, phase composition, morphology, luminescence emission, colorcoordinates, etc. using test methods that are used for conventionallyprepared phosphors.

EXAMPLE 1

Halophosphor

a. Pyrophosphate Synthesis

It was found that the conversion from CaHPO₄ (dicalcium phosphate) toCa₂P₂O₇ (pyrophosphate) could be completed within 10 minutes at 450° C.by microwave processing. The weight loss and X-ray diffraction (XRD) ofthe product indicated that the conversion reaction was complete. Themicrowave converted powders were pure β-pyrophosphate and were finerthan the conventional powder. When the temperature or treating time wasincreased, the particle size of the product increased. However, themicrowave converted pyrophosphate powders were obviously finer than theconventionally converted product (See Table 1). The finer powder will bemore reactive and thus facilitate the synthesis reaction. Pyrophosphatecan be used for the synthesis of halophosphate phosphors.

TABLE 1 Median particle size of the microwave converted pyrophosphateConventional Microwaved Starting Material 800° C./20 min. 450° C./35min. 450° C./20 min. 7.58 μm 6.40 μm 6.15 μm 5.75 μm

b. Calcium Halophosphate Synthesis

The microwave processing technique was used for the synthesis of ahalophosphor (calcium chlorofluoroapatite doped with Sb³⁺ and Mn²⁺) froma mixture of raw materials. In order to simplify the synthesis process,dicalcium phosphate (HCaPO₄) was used in the starting mixture instead ofpyrophosphate (Ca₂P₂O₇). Thus the microwave synthesis included both apyrolysis and the formation of chlorofluoroapatite in a one-stepsynthesis. The raw starting mixture was composed of HCaPO₄, CaCO₃, CaF₂,NH₄Cl, MnCO₃, and Sb₂O₃. Typically, the halophosphate was synthesized bymicrowave processing at about 1000° C. for about 20 minutes. Themicrowave-synthesized phosphor showed the same phase, morphology, dopantincorporation, and properties as the control. It was found that using ashallow tray load of the starting mixture and a pure nitrogen flowhelped improve the quality of the product.

In a typical schedule, microwave synthesis took 20 minutes soaking atthe peak temperature, and 120 minutes total in heating. Microwaveprocessing took only a fraction of the time necessary for conventionalprocessing (typically 8–12 hours), thus providing opportunities forsaving time and energy and increasing productivity.

Table 2 provides a comparison of X and Y color coordinates of theemission from a microwave-synthesized halophosphate phosphor comparedwith a conventional halophosphate phosphor. These phosphors were excitedby 254 nm UV.

TABLE 2 C.I.E. color coordinates on a microwave-synthesizedhalophosphate phosphor sample Control Microwaved sample X color 0.40560.3951 Y color 0.4091 0.4112

EXAMPLE 2

BAM

Conventionally, BaMgAl₁₀O₁₇:Eu (BAM) is produced by firing the mixtureof raw materials in a reducing atmosphere at 1600–1650° C. for two hourswith BaF₂ as flux. The total firing process lasts 6–8 hours. In theconventional production of BAM, alumina crucibles are used in a pusherfurnace. Each crucible can be used only for a limited number of runs.

a. BAM with Moderate Flux Level

Mixtures containing aluminum hydroxide, magnesium oxide, bariumcarbonate, and europium oxide with a moderate level of BaF₂ flux (10%)were fired in the microwave furnace. Temperatures ranged from about1250° C. to about 1500° C. in an atmosphere of 25% N₂ and 75% H₂ for upto about 20 min. The fired phosphor remained loose, soft, and fine (SeeTable 3).

TABLE 3 Processing conditions and physical properties of the microwaveBAM with flux Run No. Temp. (° C.) Atm. Hold, min. Powder/LOI XRD BAM 61500 25%H₂ 2 loose/30.77 identical to control BAM 7 1500 25%H₂ 10loose/30.72 identical to control BAM 8 1400 25%H₂ 10 loose/30.72identical to control BAM 9 1350 25%H₂ 20 loose/27.43 identical tocontrol

Below 1350° C., a second phase appeared in the as-fired materials as aminor phase, thus the reaction was not 100% complete. At about 1400° C.or above, the reaction was complete. This was confirmed by both the losson ignition (LOI) and x-ray diffraction patterns. Allmicrowave-synthesized phosphors were easily screened to 160 μm withoutgrinding. Table 4 lists the median particle size measured by Malvern andBET surface area of several microwave-synthesized BAM phosphors.Compared to the BET of 0.8–1.2 m²/g generally obtained by theconventional process in industry, the microwave-synthesized powders wereobviously finer. In addition, microwave processing of BAM lowered theprocessing temperature by 200–250° C. and the processing time at peaktemperature by 83% (20 minutes vs. 2 hours).

TABLE 4 Median particle size and specific surface area of themicrowave-synthesized BAM phosphors with flux Median Particle SurfaceArea Run No. Conditions Atm Size (Malvern) μm (BET) m²/g BAM 4 1470° C.× 5 min 100%N₂ 7.41 0.62 BAM 5 1425° C. × 20 min 25%N₂:75%H₂ 4.85 1.35BAM 6 1500° C. × 2 min 25%N₂:75%H₂ 5.45 1.17 BAM 7 1500° C. × 10 min25%N₂:75%H₂ 5.08 1.36 BAM 8 l400° C. × 10 min 25%N₂:75%H₂ 5.21 1.52 BAM9 1350° C. × 20 min 25%N₂:75%H₂ 5.27 1.65

b. BAM with No Flux

Unfluxed BAM starting material was fired by microwave processing in 25%N₂/75% H₂ at temperatures between 1400–1500° C. as listed in Table 5.

TABLE 5 Microwave processing conditions and physical properties ofunfluxed BAM phosphor Temp. Run No. (° C.) Hold, min. Powder/LOI XRD BAM10 1400 15 white/loose/30.66 Same as control* BAM 11 1500 5white/loose/30.68 Same as control* BAM 12 1500 20 white/loose/30.71 Sameas control* *but with a trace extra peak at 28.5° 2θ.

As the temperature increased, the powder was whiter. The LOI increasedvery little with temperatures from 1400–1500° C. In all themicrowave-fired unfluxed BAM phosphors, there was a tiny peak at 28.5deg. 2-theta of XRD pattern. For the unfluxed BAM mixture, the firingtemperature in microwave synthesis should be above 1500° C. for the bestreaction.

X-ray diffraction patterns were obtained for several samples ofmicrowaved BAM phosphors. As shown in FIG. 5, the patterns of phosphorsboth with and without flux are identical to the pattern of aconventional BAM.

EXAMPLE 3

LAP

Conventionally, LAP is prepared using fluxes such as boric acid (H₃BO₃)and lithium carbonate (Li₂CO₃) and is fired at 1200° C. for about fourhours in a reducing atmosphere. The microwaved LAP phosphors wereprepared from a mixed co-precipitate of lanthanum phosphate, ceriumphosphate, and terbium phosphate. LAP phosphors with and without fluxwere synthesized. The microwave time and temperature conditions werevaried (See Table 6). Preferably, the LAP phosphors are synthesized atabout 800° C. to about 1125° C. for about 10 minutes to about 30minutes.

TABLE 6 LAP phosphors synthesized by microwave processing Run No. TypeConditions Atmosphere LAP 1 Unfluxed 1125° C./30 min 5%H₂/Ar LAP 2Fluxed 1100° C./10 min 5%H₂/Ar LAP 3 Unfluxed 900 ± 50° C./20 min5%H₂/Ar LAP 4 Unfluxed 965 ± 65° C./20 min 5%H₂/Ar LAP 5 Unfluxed 910 ±50° C./20 min Static Air LAP 6 Unfluxed 970 ± 75° C./20 min Static AirLAP 7 Fluxed 885 ± 75° C./10 min 5%H₂/Ar

These LAP phosphors were characterized by measuring the powderbrightness compared with a conventional LAP phosphor. The medianparticle size was measured using a Malvern instrument before and aftersonification. The span is a measure of the particle size distribution(See Table 7).

TABLE 7 Properties of LAP phosphors synthesized by microwave processingRun % Median Particle Size Median Particle Size No. Brightness (SonicMalvern) (Non-Sonic Malvern) Span LAP 1 103.5 5.40 5.92 1.344 LAP 2102.4 5.17 5.58 2.417 LAP 3 100.0 4.69 5.66 1.115 LAP 4 103.5 4.51 5.591.252 LAP 5 95.4 4.81 5.63 1.237 LAP 6 99.0 4.52 5.57 1.133 LAP 7 96.64.84 8.71 2.448

The thermal stability of several microwaved LAP phosphors was measuredand compared with a conventional LAP phosphor. It was observed that theLAP made from unfluxed starting material showed better thermal stabilitythan the LAP made from fluxed starting material, and the LAP made fromstarting material fired at a lower temperature showed better thermalstability than one made from starting material fired at a highertemperature. As shown in FIG. 6, the phosphor microwaved at 1020° C. for10 minutes showed the best thermal stability.

Eight LAP phosphors (65 g per run on average) were fired by microwaveprocessing under the same conditions with tight control of the heatingprocess. The designed condition was 10 minutes at 1020° C. under flowing5% H₂/Ar. The firing process was very stable for all the startingmaterials. The total heating process in the microwave chamber (includingwarm-up and hold) was about 90 minutes. All the resultant phosphors werewhite, clean, loose, and fine powders. These phosphors were preparedwithout any flux. The properties of these LAP phosphors are listed inTable 8. All these LAP phosphors again, showed excellent properties. Thethermal stability of this batch was even better: there was nodegradation up to 600° C. The low standard deviation between runs alsoindicated the excellent uniformity and reproducibility of the process.

TABLE 8 Properties of the microwave-synthesized LAP phosphors MedianParticle % Size Temp Run No. Brightness X Y (Malvern) Span Flux Min (°C.) LAP 8 104.8 0.341 0.577 5.89 1.604 No 10 1022 LAP 9 105.2 0.3400.577 5.33 1.248 No 10 1020 LAP 10 106.2 0.339 0.578 5.48 1.299 No 101030 LAP 11 105.3 0.340 0.577 5.65 1.182 No 10 1020 LAP 12 104.4 0.3400.577 5.55 1.137 No 10 1019 LAP 13 106.3 0.342 0.578 5.57 1.138 No 101034 LAP 14 105.6 0.340 0.577 5.45 1.211 No 10 1030 LAP 15 107.3 0.3400.578 5.71 1.275 No 10 1021 Avg 105.6 0.3403 0.5774 5.58 1.262 10 1025Std Dev 0.93 0.0009 0.0005 0.17 0.151 0 5.86

The unfluxed LAP phosphor can be made by microwave processing at about1020° C. with a 10-minute hold, whereas the fluxed LAP phosphor can bemade at an even lower temperature within minutes. Compared to theconventional firing, microwave processing can save processing hold timeby more than 95% and yet at a lower temperature. Microwave processingmakes it possible to synthesize a LAP phosphor of high quality withoutusing any flux.

EXAMPLE 4

YOE

The conventional process of producing YOE requires the firing of the rawmaterials at a maximum of 1300° C. in air for up to seven hours. Thestarting material is a mixed co-precipitate of yttrium oxide andeuropium oxide. Fluxes are normally used in this firing and can consistof lithium carbonate, potassium carbonate, and boric acid. Several YOEphosphors were prepared by microwaving the starting material at a rangeof times and temperatures. Both fluxed and unfluxed phosphors wereprocessed. The starting materials were fired at temperatures from about1100 to about 1350° C. at times from about ten minutes to about fortyminutes.

X-ray diffraction and emission tests were conducted on several unfluxedYOE phosphors synthesized in air by microwave processing at 1250–1325°C. (See Table 9). As shown in FIG. 7, the XRD patterns of themicrowave-synthesized YOE phosphors are identical to the control. Asshown in FIG. 8, the emission property under 254 nm excitation intensityvaries with the firing conditions.

TABLE 9 Microwave firing conditions of unfluxed YOE phosphors Run No.Conditions Atm YOE 1 1250° C./30 min Air YOE 2 1300° C./30 min Air YOE 31325° C./30 min Air YOE 4 1325° C./40 min Air

The morphology of the co-precipitate and the microwave-synthesizedunfluxed YOE phosphors were studied by SEM. Compared with the unfiredco-precipitate, that is loose and jagged, the microwave-synthesizedphosphors showed bonding and coalition between primary particles,indicating that sintering has taken place during the short time ofmicrowave processing.

A series of fluxed YOE phosphors were synthesized by microwave firing at1200° C. for 20 minutes in air. The microwave synthesized YOE phosphorswere softer than the conventional products. About 500 grams YOE phosphorpowder was obtained by mixing several fluxed YOE phosphors synthesizedby batch microwave processing. The mixed sample was washed in the sameway as for the commercial products, and then used for a lamp test withconventionally synthesized commercial YOE phosphor as the control. Itwas found that the 100-hour lumens of the microwave synthesized YOE was99.7% of the control. The 100-hour maintenance of the microwavesynthesized YOE was 99% of the original. The microwave synthesized YOEunder the above conditions showed comparable properties to thecommercial product. Table 10 shows the firing conditions in microwaveand conventional processing. Note, that in the microwave synthesis ofthese YOE phosphors, the firing time at the peak temperature was lessthan 5% of the conventional process, yet the temperature in microwaveprocessing was 100° C. lower than that in the conventional process. Thisproves that microwave synthesis of phosphors can be commercialized toproduce high quality products with substantial time and energy savings.

TABLE 10 Comparison of microwave synthesized fluxed YOE withconventional product Process Conditions 100-h lumens in lamp testConventional 1300° C. × 7 hours, air Standard Microwave 1200° C. × 20min, air 99.7%

The morphology of unfluxed microwaved YOE is different from theconventional YOE phosphors synthesized with flux, in which particleshave grown and have rounded edges. However, phosphors that weremicrowaved with flux show similar morphology to the conventional YOE.Compared to conventional firing, YOE phosphors can be synthesized bymicrowaving in much shorter times. This microwave process has thepotential to save energy and time.

The present invention has been described with reference to the preferredembodiments. Obvious modifications, combinations, and alterations willoccur to others upon reading the preceding detailed description. It isintended that the invention be construed as including all suchmodifications, combinations, and alterations insofar as they come withinthe scope of the appended claims or the equivalents thereof.

1. A method for obtaining phosphors through microwave synthesis, themethod comprising the steps of: providing an insulator having an openingtherein, wherein the opening is substantially symmetrically disposed inrelation to a central axis of the insulator, and wherein the opening isadapted to receive a phosphor starting material; depositing the startingmaterial within the opening; and subjecting the phosphor startingmaterial to microwaves.
 2. The method of claim 1, further comprising thestep of positioning a susceptor configuration within the cavity, whereinthe susceptor configuration is substantially symmetrically disposed inrelation to the central axis of the insulator.
 3. The method of claim 1,wherein the insulator is comprised of refractory material that isessentially transparent to microwaves in a working temperature range. 4.The method of claim 3, wherein the insulator is comprised of one ofaluminosilicate fibers and alumina fibers.
 5. The method of claim 4,wherein the phosphor is one of halophosphate, barium-magnesiumaluminate, lanthanum phosphate, and europium doped yttrium oxide.
 6. Themethod of claim 1, further comprising the step of positioning asusceptor configuration within the cavity.
 7. The method of claim 1,wherein the microwave furnace is adapted to be tilted by raising thefirst end of the tube to facilitate the movement of the startingmaterial through the tube.
 8. A method of synthesis of a phosphor bymicrowave processing, the method comprising the steps of: (a) providinga microwave furnace having a microwave chamber; (b) providing a phosphorstarting material in the microwave chamber; and (c) subjecting thephosphor starting material to microwaves, whereby the starting materialis synthesized into a phosphor.
 9. The method of claim 8, wherein thephosphor is a halophosphate phosphor.
 10. The method of claim 9, whereinthe phosphor is represented by Ca₁₀(PO₄)₆(Cl,F):Sb:Mn.
 11. The method ofclaim 9, wherein the phosphor is synthesized by microwave processing atabout 1000° C.
 12. The method of claim 10, wherein the starting materialcomprises a mixture of HCaPO₄, CaCO₃, CaF₂, NH₄Cl, MnCO₃ and Sb₂O₃. 13.The method of claim 12, wherein the phosphor is synthesized by microwaveprocessing at about 1000° C. for about 20 minutes.
 14. The method ofclaim 8, wherein the phosphor is a barium-magnesium aluminate.
 15. Themethod of claim 14, wherein the phosphor is represented byBaMg_(1+x)Al_(10+y)O_(17+z), wherein 0<x<2.0, 0<y<5.0, and 0<z<10.5 andthe phosphor has a europium activator.
 16. The method of claim 14,wherein the phosphor is synthesized by microwave processing in areducing atmosphere at about 1250° C. to about 1500° C.
 17. The methodof claim 16, wherein the phosphor is synthesized by microwave processingat about 1400° C. to about 1500° C.
 18. The method of claim 15, whereinthe starting material is a mixture of aluminum hydroxide, magnesiumoxide, barium carbonate, europium oxide and a barium fluoride flux. 19.The method of claim 18, wherein the phosphor is synthesized by microwaveprocessing in a reducing atmosphere at about 1250° C. to about 1500° C.for about 20 minutes.
 20. The method of claim 8, wherein the phosphor isa lanthanum phosphate.
 21. The method of claim 20, wherein the phosphoris represented by (La, Ce, Tb)PO₄:Ce:Tb.
 22. The method of claim 20,wherein the phosphor is synthesized by microwave processing at about800° C. to about 1125° C.
 23. The method of claim 21, wherein thestarting material is a coprecipitate mixture of lanthanum phosphate,cerium phosphate and terbium phosphate.
 24. The method of claim 21,wherein the starting material further includes a flux.
 25. The method ofclaim 23, wherein the phosphor is synthesized by microwave processing atabout 800° C. to about 1125° C. for about 10 to about 30 minutes. 26.The method of claim 8, wherein the phosphor is a europium doped yttriumoxide.
 27. The method of claim 26, wherein the phosphor is synthesizedby microwave processing at about 1100° C. to about 1350° C.
 28. Themethod of claim 26, wherein the starting material is a mixture ofyttrium oxide and europium oxide.
 29. The method of claim 28, whereinthe phosphor is synthesized by microwave processing at about 1100° C. toabout 1350° C. for about 10 minutes to about 40 minutes.
 30. The methodof claim 29, wherein the mixture further includes a flux.
 31. The methodof claim 8, wherein the phosphor is synthesized without a flux.